Steel which does not rust, frequently also referred to as stainless steel, is an iron alloy which in addition to iron and chromium may also contain further elements such as nickel, molybdenum, titanium, copper and others. An essential constituent of the stainless steel alloys whose treatment comprises part of the subject matter of the present invention is the element chromium which is present at a minimum concentration of about 12% by weight in order that enhanced resistance of the steel to corrosion may be ensured. The chromium present in the alloy reacts at the surface with oxygen from the surroundings to form a layer of oxide on the surface of the workpiece material. From a chromium content of about 12% by weight in the alloy of the workpiece material concerned, the chromium oxide formed is consistently able to form an impervious layer on the surface and thus protects the workpiece against corrosion. This protective layer is known as a passive layer.
Such a passive layer is generally about 10 molecular layers in thickness and, in addition to chromium oxide, contains particularly iron oxide at a concentration of 10-55% by weight. The lower the proportion of iron oxide in the passive layer, the higher the chemical resistance of the layer.
A thermal treatment of stainless steel in an oxidizing atmosphere at temperatures above 200° C. brings about a progressive thermal oxidation of the workpiece material to form an oxide layer consisting essentially of oxides of the metals present in the alloy and whose quantitative ratio between the oxides corresponds essentially to the quantitative ratio between the metals in the alloy. The thermally produced oxide layers therefore contain up to about 87% by weight of iron oxides, depending on the alloy. These oxide layers grow in thickness with increasing temperature and treatment time and lead to discolorations through to black or gray coatings. These are known as scale and annealing/tempering colors.
Oxide layers of this type, which contain distinctly more iron oxide than chromium oxide, are not resistant to corrosion, and so the stainless steel in these regions is not sufficiently corrosion-resistant for general use. In moist surroundings, the iron oxide reacts with water to form iron hydroxide and rust.
Consistent and complete removal of scale and annealing/tempering colors off stainless steel surfaces is an absolute prerequisite for the subsequent formation of an intact passive layer which is responsible for the corrosion resistance of stainless steel. In the prior art, thermal oxides are removed either by mechanical cleaning via grinding, brushing or particle blasting or by chemical or electrolytic pickling. The mechanical methods have the disadvantage that their cleaning effect is incomplete and insufficient and does not reach difficult-to-access regions such as corners, slots and cavities. And small and sensitive workpieces are easily damaged.
Electrolytic pickling utilizes aqueous mixtures of mineral acids which via agency of direct current lead to an anodic ablation of the uppermost layer of metal through electrochemical dissolution which also removes the oxide layers on top. These methods can only be applied in the case of thin layers of oxide which are pervious to direct current and electrolyte. They further require an appreciable capital investment in plant technology. They employ hazardous substances and generate wastewaters comprising heavy metal which are costly and inconvenient to treat and dispose of.
Chemical methods of pickling dissolve the oxide layers and the metal of the uppermost layer of workpiece material chemically to produce a metallically clean surface. A homogeneous passive layer can subsequently be formed on this metallically clean surface to protect the workpiece material efficaciously against corrosion. Chemical pickling allows the entire surface of workpieces to be treated, including difficult-to-access regions. What is disadvantageous is the fact that dissolving the oxides and the workpiece material requires extremely aggressive and hazardous chemicals which represent a considerable risk to humans and the environment.
Essential constituents of chemical pickles for stainless steel are hydrofluoric acid (HF) or fluorides of salts of hydrofluoric acid, which form hydrofluoric acid in aqueous solution, and also oxidizing agents such as nitric acid or hydrogen peroxide. Hydrofluoric acid is extremely poisonous in that even relatively minimal contact with the skin can be fatal. Nitric acid when used in pickling releases poisonous nitrous gases which are very harmful to the lung. Hydrofluoric acid and nitric acid are fuming acids, and so the air in the workplace environment has to be aspirated and specially treated. The personnel deployed in chemical pickling has to wear appropriate protective clothing with or without a respirator, and is subject to constant medical monitoring.
There are strict safety regulations governing the production, transportation, storage and use of the chemicals used for chemical pickling. The wastewaters generated in chemical pickling contain high concentrations of the acids in the pickle and also of the heavy metals in the alloy, such as chromium, iron, nickel and molybdenum. They require costly and inconvenient chemical treatment for disposal, and the solids generated have to be landfilled as special waste. Spent pickling solutions have to be disposed of as hazardous special waste.
Given the greatly increasing use of stainless steel in all areas of everyday life and industry, and the attendant increasing need for pickling, there is an urgent demand for a method which in terms of performance is comparable to the pickling method for stainless steel, but is harmless to humans and the environment.
The present invention provides a chemical stainless steel surface treatment method which is harmless to humans and the environment and in terms of the achievable corrosion resistance at least equivalent, but largely distinctly superior, to the prior art processes in respect of the corrosion resistance which is obtainable.